Tire parameter sensing systems for vehicles typically include a plurality of tire-based units and a single vehicle-based unit. Each tire-based unit has an associated tire of the vehicle and is operative to sense at least one parameter of the tire. The sensed parameter(s) may include temperature, pressure, etc. Each tire-based unit is also operative to transmit a parameter signal indicative of the sensed parameter(s) to the vehicle-based unit. The vehicle-based unit is connected to a display. In response to receiving a parameter signal from a tire-based unit, the vehicle-based unit outputs a signal to the display. The display is responsive to the signal for displaying the sensed tire parameter(s).
It is common for the tire-based units of a tire parameter sensing system to be battery powered. Battery powered tire-based units, however, have specific limitations, such as, for example, a limited life, a limited current supply, and a limited operating temperature range. The design of a tire parameter sensing system using battery powered tire-based units must be mindful of these limitations. As a result, it is common for a battery powered tire-based unit to transmit parameter signals only in response to a determination that a sensed parameter is outside of a desired range. For example, if the desired pressure range is 32 to 36 pounds per square inch (“psi”), the battery powered tire-based unit may transmit a parameter signal to the vehicle-based unit only when the sensed tire pressure is determined to be below 32 psi or above 36 psi. By limiting the transmissions of the parameter signal, the battery life of the battery powered tire-based unit may be extended.
In some tire parameter sensing systems, the tire-based units do not include batteries. Tire-based units that do not include batteries may receive energy through induction. When the tire-based units receive power through induction, devices for producing a magnetic field are located adjacent the tires having the tire-based units. Each tire-based unit includes an antenna in which electrical energy is induced when the antenna is placed in the magnetic field. In order for a sufficient amount of electrical energy to be induced in the antenna at all rotational positions of the tire-based unit relative to the magnetic field producing device, the antenna typically is a loop antenna that extends completely, around the circumference of the tire.
U.S. Pat. No. 6,470,933 discloses a system in which a bead of a tire forms the antenna of the tire-based unit. The bead of the tire extends completely around the circumference of the tire so that electrical energy will be induced at any rotational angle of the tire relative to the vehicle.